Originally Posted by
Raffles
They won't cancel your Avios. They may, if done repeatedly, invoice you for the 'proper' fare for the route you actually flew. Only serial abusers are at any risk, though.
Quite the opposite. They are almost certain to remove the Avios earnt for any sectors flown which have have deemed to have broken the rules of the BAEC. They could also go after you directly for a difference in fare or to send a debit memo to your travel agent. I agree though, that the latter is likely only for serial abusers.
I'm not suggesting I agree with any of this - but this is what has known to have happened before.